Changing module files

[Andrew Benson]

I'm finding strange behavior in which a module file generated on compiling a 
source file sometimes changes, even though the source file (and everything else) 
is unchanged. The change seems to be of a binary nature - running md5sum on 
the module file returns one of two values. The module files seem to be almost 
identical - usually with some lines moved and a few numbers changed. 
Everything still compiles correctly, but the changing module files make it 
impossible to do dependency checks in a Makefile when compiling a large project 
(e.g. sometimes a source file can receive a minor change - one which shouldn't 
normally affect the module file - gets recompiled producing a different module 
file which then triggers a recompile of a large number of dependent files).

This is happening with gfortran 4.7.0 r182342.

Unfortunately, I haven't been able to find a self-contained test case as the 
problem is intermittent and seems to be very sensitive to the structure of the 
source file. Additionally, it doesn't even seem to happen on every system that 
I've tried it on - the problem occurs on a RHEL5 Linux system, but not on a 
Fedora 16 system (both running the exact same version of gfortran). The best I 
can offer is a tarball (attached) containing the source file and various module 
files which is USEs. I've included a script, test.sh, which compiles the source 
100 times and reports the md5sum of the module. The reported value should show 
two different values - one reported about 90% of the time and the other about 
10% of the time - seemingly at random.

If anyone has suggestions of how to get a better test case I'd be happy to 
pursue this further.

-Andrew.

-- 

* Andrew Benson: http://www.tapir.caltech.edu/~abenson/contact.html

* Galacticus: http://sites.google.com/site/galacticusmodel
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